(1) Field of the Invention
The present invention relates to a three-layer composite antistatic filament comprising layer including electrically conductive carbon black dispersed therein and a process for the preparation thereof.
(2) Description of the Prior Art
As known antistatic composite filaments having a layer containing electrically conductive carbon black, the following filaments can be mentioned.
(A) A two-layer composite filament comprising a core layer having electrically conductive carbon black dispersed therein and an electrically non-conductive sheath layer (see Japanese Patent Publication No. 31450/77).
(B) A two-layer composite filament comprising a sheath layer containing electrically conductive carbon black and an electrically non-conductive core layer (see Japanese Patent Application Laid-Open Specification No. 48715/73).
(C) A composite filament comprising an electrically conductive layer having electrically conductive carbon black dispersed therein and an electrically non-conductive layer, in which said two layers are bonded to each other asymmetrically (see Japanese Patent Application Laid-Open Specification No. 143723/76).
Cross-sections of typical instances of these known composite filaments are shown in FIG. 1. In the filament shown in FIG. 1-A, an electrically conductive layer 2 constitutes the core and an electrically non-conductive layer 1 constitutes the sheath. In the filament shown in FIG. 1-B, an electrically non-conductive layer 1 constitutes the core and an electrically conductive layer 2 constitutes the sheath. In the filament shown in FIG. 1-C, electrically conductive layer 2 and electrically non-conductive layer 1 are bonded to each other asymmetrically.
In these antistatic composite filaments, a synthetic polymer having finely divided, electrically conductive carbon black dispersed therein at a high concentration is used so as to impart an intended low electric resistance to the final composite filament. This carbon black-dispersed synthetic polymer is, however, very poor in the yarn forming property and a filamentary yarn having practical properties cannot be obtained from this polymer. Accordingly, this carbon black-dispersed synthetic polymer should be composite-spun or combined with a synthetic polymer excellent in the fiber forming property to improve the yarn forming property of the carbon black-dispersed synthetic polymer and obtain a filamentary yarn having a sufficient strength.
In the case of the composite filament shown in FIG. 1-A, in order to improve the fiber forming property and yarn forming property sufficiently, the thickness of the sheath should inevitably be increased and therefore, the electric conductivity given by electrically conductive carbon black dispersed in the core should inevitably be reduced to a low level. For example, in Japanese Patent Application Laid-Open Specification No. 143723/76 it is pointed out that the composite filament shown in FIG. 1-A is not effective at all for eliminating static charges below the level of 3,500 volts, which is ordinarily sensed by men. Accordingly, the composite filament of the type shown in FIG. 1-A involves a problem of a low antistatic effect.
In the case of the composite filament shown in FIG. 1-B, since a carbon black-containing layer is present as the outermost layer, the black color inherent of carbon black is very conspicuous in a yarn formed from such filament, and if this yarn is used for the manufacture of a knitted or woven fabric or a carpet, the product inevitably has an undesirable appearance. Moreover, if this composite filament undergoes a bending or frictional action during subsequent processing steps, carbon black present in the outermost layer is peeled or taken out and there is brought about a defect of reduction of the antistatic effect.
Also in the filament shown in FIG. 1-C, the layer of a polymer having electrically conductive carbon black dispersed therein is exposed to the surface of the filament, though not along the entire periphery of the filament. Accordingly, this composite filament involves a problem of a poor appearance as well as the filament shown in FIG. 1-B. Moreover, this filament has a defect that black pieces are readily caused to fall down. Moreover, this composite filament involves fundamental defects inherent of a bonded type composite filament. More specifically, bending is caused on the surface of a spinneret at the spinning step, and crimps are readily formed when the filament is drawn.
In each of the filaments shown in FIGS. 1-B and 1-C, since a carbon black-containing layer is not present in the interior but is exposed directly to the filament surface. Accordingly, in order to attain a good fiber forming property in this layer, the content of carbon black should be controlled below a certain upper limit. Furthermore, if a yarn of this composite filament is subjected to subsequent processing such as false twisting, fusion bonding of filaments is caused unless the melting or softening point of the synthetic polymer containing carbon black is lower than the melting or softening point of an electrically nonconductive filament to be combined therewith.